Against this background, it is apparent that, while some products have reconciled these competing “must-haves” to some extent, there has been a longstanding un-met need for a bath tissue truly surmounting these inherent conflicts. There are also many patents that seem to assume that there is little more to making a viable bath tissue that is usable either dry or premoistened than providing a tissue weight product with significant initial wet strength that decays with time. We have discovered that we can provide a bath tissue that achieves a previously unmatched combination of wet and dry properties by incorporating a minor proportion of cellulosic microfibers into a furnish that is used for bath tissue, then forming a tissue web using a belt creping process, in which a nascent web at a consistency of between about 30 and about 60% is creped from an internally heated creping roll using a creping belt. We have found that, with belt-creping and cellulose microfiber (CMF) content, we can produce sheets that are particularly resistant to linting, even when used wet, while also retaining both sufficient wet strength to protect the user's hand and sufficient softness to be used dry by sensitive users. We have also found that we can substitute a controlled coarseness alkaline peroxide mechanical pulp (APMP) into these wet strength bath tissues as a replacement for eucalyptus kraft, and obtain excellent softness, wet strength, lint resistance and wet lint resistance, with very small amounts of CMF.
Others have attempted to address the need for a flushable bath tissue that can be used dry or premoistened, with a variety of wet wipe formulations, wherein fibers in the wet wipe are bonded together with a strength resin that is stabilized by a chemical species in the imbuement of the wet wipe, but is destabilized upon exposure to a large quantity of “normal” water. Thus, the wipe remains strong as long as the imbuement is in contact with the wipe in its package and for a period after removal, because the imbuement stabilizes the resin, but, when the imbuement is removed or, more properly, diluted with water, the strength agent is rendered less effective and the wipe, at least in theory, becomes dispersible. A major tissue producer is reported to have attempted to commercialize such a technology that was, however, not well accepted by the market. In another approach involving wet wipes, some circles maintain that flushability does not really require that the sheet disintegrate after flushing as long as the size of the sheet is kept under some fairly small maximum. Limited consumer research that has been conducted, however, indicates that most users will remove and use several sheets of bath tissue from the roll at a time, so that the sheet used has an area several times that of the so-called “flushable wet wipe” in which “flushability” is predicated upon the size of the sheet.
In contrast, the present invention is based upon a product that can be stored dry on a perforated roll and used like a conventional bath tissue employing a convenient number of unseparated sheets, as the user prefers. If, however, premoistened shortly before use, the tissue retains a sufficient wet strength to be used in the moist state without linting, pilling or shredding, but will disintegrate within a reasonable time after flushing, so that the effectiveness of plumbing is not unduly compromised. Significantly, these goals are achieved without requiring another product in the bathroom; although some users may prefer to use a small container, such as a spray bottle to hold aqueous liquid for moistening the sheet immediately before use. Such bottles can be conveniently disposed inside the roll core of packaged tissue, as promotional or introductory items, if desired.
As use of the present invention makes it possible to achieve quite a high wet/dry ratio, softness does not unduly suffer, as the actual dry tensile strength, which is strongly associated—negatively—with perceived softness, can be moderate, while the wet strength can remain quite high in the first minutes after moistening. Thus, the strength and softness of the tissue of the present invention can be comparable to that of premium bath tissue, while retaining a high temporary wet strength. Even though, when used with conventional papermaking furnishes, many wet strength resins make it possible to achieve wet strength levels necessary for the tissue to be employed premoistened, in many cases, the tactile properties of the dry sheet can be somewhat compromised thereby.
When employing substantial amounts of cellulosic microfibers in the furnish, in conjunction with temporary wet strength resin and belt-creped paper making technology, we have found that we can achieve a surprisingly good combination of softness, opacity, wet strength and resistance to pilling and shredding in a flushable bath tissue. In addition, the flushable bath tissue is capable of being stored on a roll, as is conventional bath tissue, and suitable for use either dry or premoistened. When alkaline peroxide mechanically pulped (APMP) eucalyptus fiber is included, we have found that we can obtain excellent results, even using far less of the cellulosic microfiber, even when using conventional wet press (CWP) technology. We have further discovered that the APMP eucalyptus fiber is an excellent substitute for conventional eucalyptus kraft fiber in conventional bath tissue, imparting increased opacity, bulk, softness, absorbency and reduced strength, even to tissue made with recycle furnishes.
One early pre-wettable tissue was disclosed in Bhat et al., “Prewettable High Softness Paper Product Having Temporary Wet Strength”, U.S. Pat. No. 5,958,187, Sep. 28, 1999, relating to a paper product with a glabrous surface and adapted for use either dry or use in a manually pre-moistened condition. Bhat et al. disclose a paper product having a temporary wet strength and exhibiting an initial normalized cross machine direction (CD) wet tensile strength of at least about 25 g/in. strip, preferably, 35 g/in. strip as measured by the Finch Cup Test 5 seconds after immersion, and a subsequent CD wet tensile strength of less than about ⅔ the initial value as measured 30 minutes after immersion. Temporary wet strength was provided by addition to the furnish of a temporary wet strength agent comprising aldehydic units in the range of from about 2 pounds per ton to about 30 pounds per ton. The furnish also included a cationic nitrogenous softener/debonder in an amount of from about 1 pound per ton to about 6 pounds per ton. The CD dry tensile strength of the paper product was from about 133 g/in. strip up to about 267 g/in. strip, and the tensile modulus was from about 10 to about 32 g/% strain, while the geometric mean friction deviation (GM MMD value) was from about 0.26 to about 0.10. The CD wet strength of the product decays to about 15 g/in. within 10 hours after immersion. When rubbed against a skin-like surface in a moistened condition, the paper product remains substantially free of pilling. Significantly, in Bhat et al., the wet abrasion resistance of a 2″ by 4.5″ sample of tissue was measured under a load of 135 grams against a wetted pigskin, and visual observation was made to determine whether the sample left pills, shreds or lint behind.
Another early pre-wettable tissue was disclosed in Van Luu et al. [sic, Luu et al.], “Prewettable High Softness Paper Product Having Temporary Wet Strength”, U.S. Pat. No. 6,059,928, May 9, 2000, in which a temporary wet strength agent comprising uncharged chemical moieties, such as aldehydes, and aldehydes containing polymers, polyols and cyclic ureas, or mixtures thereof, in the range of from about 2 pounds per ton to about 30 pounds per ton are added to the web to provide the temporary wet strength. In this application, glyoxal was preferably sprayed on the sheet after it left the Yankee dryer.
“Belt-Creped, Variable Local Basis Weight Absorbent Sheet Prepared With Perforated Polymeric Belt” is disclosed in Super et al. U.S. Patent Application Publication No. 2010/0186913, now U.S. Pat. No. 8,293,072 (incorporated herein by reference), which produces cellulosic tissue sheets exhibiting a surprising combination of bulk, roll firmness, absorbency and softness, from a sheet with a fiber-enriched higher basis weight, hollow domed regions joined by a network of lower local basis weight connecting regions forming a network in which upwardly and inwardly inflected consolidated fibrous regions exhibiting CD fiber orientation bias form transition areas between the connecting regions and the domed regions. The consolidated fibrous regions are, preferably, saddle shaped and exhibit a matted structure on both their outer and inner surfaces. Related technology is found in the following U.S. Patent Applications and U.S. Patents: U.S. Pat. No. 7,494,563 entitled “Fabric Creped Absorbent Sheet with Variable Local Basis Weight”, U.S. Pat. No. 7,399,378 entitled “Fabric Crepe Process for Making Absorbent Sheet”, U.S. Pat. No. 7,789,995 entitled “Fabric Crepe/Draw Process for Producing Absorbent Sheet”, the application of which was a continuation-in-part of the application of U.S. Pat. No. 7,399,378 entitled “Fabric Crepe Process for Making Absorbent Sheet”, U.S. Pat. No. 7,442,278 entitled “Fabric Crepe and In Fabric Drying Process for Producing Absorbent Sheet”, U.S. Pat. No. 7,503,998 entitled “High Solids Fabric Crepe Process for Producing Absorbent Sheet With In-Fabric Drying”, U.S. Pat. No. 7,662,257 entitled “Multi-Ply Paper Towel With Absorbent Core”, U.S. Pat. No. 7,588,660 entitled “Wet-Pressed Tissue and Towel Products With Elevated CD Stretch and Low Tensile Ratios Made With a High Solids Fabric Crepe Process”, and U.S. Pat. No. 7,585,389 entitled “Method of Making Fabric-Creped Sheet for Dispensers”, U.S. Pat. No. 7,850,823 entitled “Method of Controlling Adhesive Build-Up on a Yankee Dryer”, U.S. Pat. No. 7,651,589 entitled “Process for Producing Absorbent Sheet”, U.S. Pat. No. 7,662,255 entitled “Absorbent Sheet”, and U.S. Pat. No. 7,670,457, which are each a division of the application of U.S. Pat. No. 7,442,278; U.S. Pat. No. 7,588,661 entitled “Fabric Crepe Process for Making Absorbent Sheet”, and U.S. Pat. No. 7,704,349 entitled “Fabric Crepe Process for Making Absorbent Sheet”, which are both a division of the application of U.S. Pat. No. 7,399,378, and U.S. Pat. No. 7,670,457 entitled “Process for Producing Absorbent Sheet”. The papermaking technology disclosed in the foregoing documents in this paragraph, the disclosures of which are all incorporated herein by reference in their entireties, makes it possible to form sheets with extremely high bulk stretch and absorbency.
Canadian Patent Application No. 2,095,554 in the name of William D. Lloyd, published Aug. 6, 1994, discloses that hardwood bleached chemical thermomechanical pulp (BCTMP) fibers at amounts of about 5 weight percent or greater provide a soft tissue useful for use as facial or bath tissue, but fails to disclose the degree of bleaching and chemical refining applied to his fibers and is devoid of information concerning the brightness, lignin content or Kappa number of his fibers, other than to state that the fibers contain “substantial amounts of lignin” and the that pulping yield is “about 90% or greater”. Lloyd also states that “it is not necessary to bury the BCTMP fibers in the middle of the tissue sheet by layering. Instead, the tissue sheets can be blended using a mixture of hardwood BCTMP fibers (for softness) and longer softwood fibers (for strength). If a layered tissue is preferred, the hardwood BCTMP fibers can be utilized in the outer layer(s).”